Thunderbird 2.0.0.12 (actually most of the 2.x releases) has been behaving very weirdly on both Mac OS X and Windows Vista boxes. The weirdness is slightly different on each system but has the common element of auto-deletion behavior. Here's what I'm seeing on two different Windows Vista PCs and two different Macs. The email servers are IMAP4 mail servers.

Vista PC 1: Does not move messages tagged by SPAM Assassin to the trash folder as defined in Thunderbird's settings.

Vista PC 2: Correctly moves messages tagged by SPAM Assassin to the trash folder but does not move messages judged as spam by Thunderbird's own junk mail algorithms.

Mac 1: Does not move messages tagged by SPAM Assassin to the trash folder.

Mac 2: Moves random (or so it seems) clean messages (i.e., not tagged by SPAM Assassin or Thunderbird's own filters) to Trash. Moreover, if I manually click and drag the message from Trash to the Inbox, the files are automatically moved back to Trash.

I haven't seen any of these behaviors when using Thunderbird 2.0.0.12 on Linux boxes.

I'm pretty close to dumping Thunderbird from all my Windows and Mac boxes and moving to something else (not sure what yet though). Might go to Outlook 2007 and Entourage 2008 if I don't see any freeware or FOSS email client that appeals to me.